(a) Field of the Invention
The present invention relates to a solder ball arrangement device and, more particularly, to a solder ball arrangement device for use in transferring an array of solder balls onto a mounting board to form an array of bump electrodes in a BGA (ball grid array) technique, a CSP (chip size package) technique, or a flip chip bonding technique. The present invention also relates to a method for fabrication thereof.
(b) Description of the Related Art
Bump electrodes formed by a BGA technique, a CSP technique, and a flip chip bonding technique are increasingly used in mounting a semiconductor chip onto a mounting board. In the recent advance of semiconductor integrated circuits, a large number of external pins are disposed at a smaller pitch, with increased mounting density, reduced device area and increased capacity of the integrated circuits. In the method for forming bump electrodes by using metallic solder balls, it is especially important to arrange a large number of minute metallic solder balls (hereinafter referred to as solder balls) efficiently at a small pitch. Some proposals have been made for forming the bump electrodes.
JP-A-8(1996)-25035, for example, proposes a solder ball transfer device for installing an array of solder balls on a mounting board. FIG. 1A is a schematic cross-sectional view of the proposed solder ball transfer device.
The proposed solder ball transfer device 30 comprises an alignment plate 32 having an array of holes 32a each for receiving a solder ball 31 therein and an associated ejecting pin 33 disposed for each of the holes 32a for ejecting the solder ball 31 from the alignment plate 32. A pattern for the array of the holes 32a is made to be exactly equal to the pattern of bonding pads 35 formed on a target mounting board 34. The ejecting pin 33 is slidably mounted by the alignment plate 32 for protruding from and retracting into an associated slot 32b communicated with a corresponding one of the holes 32a at the bottom thereof. The ejecting pin 33 is of a tube shape having a communicating hole 33a therein communicated with a suction pump not shown in the figure.
In the conventional solder ball transfer device 30, a solder ball 31 is first disposed in each hole 32a, with the alignment plate 32 disposed upside-down. The suction pump is then operated to attract the solder ball 31 to the tip of the ejecting pin 33 through the communication hole 33a. Subsequently, the alignment plate 32 is turned by a mechanism so that the solder balls 31 disposed on the bottom of the alignment plate 32 opposed to the top surface of a mounting board 34. The ejecting pins 33 are then ejected to thrust the solder balls 31 onto the flux 36 on the respective bonding pads 35, as shown in FIG. 1B, followed by release of the solder balls 31 from the ejecting pins 33 by stopping the suction from the suction pump and subsequent retraction of the ejecting pins 33. Thereafter, the mounting board 34 is introduced into a reflow furnace, wherein flux 36 is melted to fix the solder balls 31 onto the bonding pads 35 of the mounting board 34.
It is reported that the conventional solder ball transfer device 30 as described above has an advantage in that the array of the solder balls 31 are transferred to the mounting board 34 with excellent reliability even if the solder ball 31 attracts electrostatic charge and/or moisture. In addition, the solder balls 31 are transferred to correct locations of a mounting board 34 even with the presence of the flux 36.
The conventional solder ball transfer device has, however, a problem in that the complicated structure of the holes 32a and associated slots 32b in the alignment plate 32 raises the cost of the device, especially in the case of a high density ball pattern such as having a small pitch as low as 300 .mu.m with a large number of pads as high as 2000. If the alignment plate is formed by an etching technique for achieving the high density, the alignment plate will suffer from lack of mechanical strength due to a small thickness thereof.